1. Field of the Invention
The invention relates to a control apparatus for an internal combustion engine.
2. Description of Related Art
In an internal combustion engine, in order to exercise the exhaust gas purification performance of a catalyst provided in an exhaust passage, sensors are provided in the exhaust passage. Each of the sensors outputs an output value corresponding to the concentration of oxygen in exhaust gas. Air-fuel ratio control for correcting a fuel injection amount is executed such that the output value of each sensor becomes an output value corresponding to a target air-fuel ratio. In this air-fuel ratio control, main feedback correction is carried out with the use of the upstream-side sensor. The upstream-side sensor outputs an output value corresponding to an actual air-fuel ratio in the exhaust passage upstream of the catalyst for purifying exhaust gas. In the air-fuel ratio control, sub-feedback correction is also carried out with the use of the downstream-side sensor to calculate a sub-correction value and then to correct the fuel injection amount, which is corrected through the main feedback correction, by using the sub-correction value. The downstream-side sensor outputs an output value corresponding to an actual air-fuel ratio in the exhaust passage downstream of the catalyst. As one of correction values that constitute the sub-correction value, a learning value that compensates for a steady deviation between an output value corresponding to the actual air-fuel ratio and an output value corresponding to the target air-fuel ratio is often used.
Incidentally, there may occur variations in fuel injection amount among cylinders, and, as a result, there may occur variations in air-fuel ratio among the cylinders. For example, there is known a technique for, when there are such cylinder-to-cylinder variations, accurately controlling the air-fuel ratios that are respectively required of cylinders by changing corresponding target air-fuel ratios (for example, Japanese Patent Application Publication No. 2013-122214 (JP 2013-122214 A), or the like).
There is known that, when the air-fuel ratio of part of the cylinders, for example, deviates to the rich side with respect to the air-fuel ratios of the other cylinders, the above-described upstream-side sensor detects hydrogen because of a high concentration of hydrogen that is emitted from the cylinders and the output value of the upstream-side sensor deviates to the rich side with respect to an output value corresponding to an actual air-fuel ratio (for example, Japanese Patent Application Publication No. 2009-30455 (JP 2009-30455 A), or the like).
When the output value of the upstream-side sensor deviates to the rich side in this way, the actual air-fuel ratio is erroneously corrected to the lean side with respect to the target air-fuel ratio in the main feedback correction in the above-described air-fuel ratio control. On the other hand, hydrogen contained in exhaust gas is oxidized as the hydrogen passes through the catalyst, so the output value of the downstream-side sensor, different from the output value of the upstream-side sensor, becomes an output value corresponding to an actual air-fuel ratio. Therefore, the actual air-fuel ratio excessively corrected to the lean side through the main feedback correction is detected by the downstream-side sensor, and a sub-correction value for increasing the fuel injection amount is calculated in order to correct the actual air-fuel ratio to the rich side in the sub-feedback correction. A deviation (in this case, a lean deviation) of the actual air-fuel ratio with respect to the target air-fuel ratio due to a deviation of the output value of the upstream-side sensor is gradually suppressed by the thus calculated sub-correction value.
On the other hand, a lean deviation of the actual air-fuel ratio due to a deviation of the output value of the upstream-side sensor at the time when there are cylinder-to-cylinder variations can also be suppressed by increasing the fuel injection amount corrected through the air-fuel ratio control. In this case, the actual air-fuel ratio becomes rich as a result of an increase in the fuel injection amount. Therefore, after enrichment of the actual air-fuel ratio is started by increasing the fuel injection amount, the learning value of the sub-correction value gradually decreases and converges to an appropriate value.
Before enrichment of the actual air-fuel ratio is started by increasing the fuel injection amount, enrichment of the actual air-fuel ratio is achieved by using the sub-correction value, and the learning value that compensates for a steady deviation between the output value of the downstream-side sensor and the output value corresponding to the target air-fuel ratio is a relatively large value. Therefore, just after enrichment of the actual air-fuel ratio is started by increasing the fuel injection amount in the state where the learning value is a relatively large value, there is a possibility that the fuel injection amount is excessively increased and, as a result, the actual air-fuel ratio becomes excessively rich. The above excessive enrichment of the actual air-fuel ratio is eliminated as the learning value of the sub-correction value gradually decreases and converges to an appropriate value as described above. However, a certain time is required for the learning value to converge to an appropriate value, so there is a concern that the exhaust gas purification performance decreases until the learning value converges to an appropriate value.
Similarly, when the actual air-fuel ratio becomes rich because of a deviation of the output value of the upstream-side sensor at the time when there are cylinder-to-cylinder variations, a rich deviation of the actual air-fuel ratio can be suppressed by reducing the fuel injection amount corrected through the air-fuel ratio control. However, in this case as well, before leaning of the actual air-fuel ratio is started by reducing the fuel injection amount, leaning of the actual air-fuel ratio is achieved by using the sub-correction value, and the learning value that compensates for a steady deviation between the output value of the downstream-side sensor and the output value corresponding to the target air-fuel ratio is a relatively large value. Therefore, just after leaning of the actual air-fuel ratio is started by reducing the fuel injection amount in the state where the learning value is a relatively large value, there is a possibility that the fuel injection amount is excessively reduced and, as a result, the actual air-fuel ratio becomes excessively lean. The above excessive leaning of the actual air-fuel ratio is eliminated as the learning value of the sub-correction value gradually decreases and converges to an appropriate value as described above. However, a certain time is required for the learning value to converge to an appropriate value, so there is a concern that the exhaust gas purification performance decreases until the learning value converges to an appropriate value.